An echo sounding technique in the ocean has been used for long. As shown in FIG. 1, an ultrasonic pulse is generated from an ultrasonic transducer, an echo obtained when its sound wave is reflected from a target (sea bottom) is captured, and its depth is measured by using a propagation speed (about 1500 m/sec) of the sound wave in the water. An echo sounding apparatus using such a principle has been realized as a product 50 or more years ago. Even nowadays, a depth of sea bottom is measured by using such a principle. Such a technique called “echo location” has continuously been used without being changed, in other words, without being developed.
Such a principle is as follows. An ultrasonic pulse (for example, a pulse width of 1 msec) is generated. In the case of the sea bottom of 500 m, now assuming that an underwater velocity Vu of the sound wave is equal to 1500 m/sec, the sound wave is returned after (1000/Vu=1000/1500=0.667 second). Therefore, after its echo was received, an ultrasonic pulse is again generated and a sea bottom at a location which differs by a distance where a ship progressed simultaneously is measured. Such an apparatus that a depth of the sea bottom is sequentially measured in association with the navigation of the ship as mentioned above and measured depths are recorded onto recording paper or displayed as images onto a liquid crystal display screen is called “echo sounding apparatus” (for example, refer to Patent Literature (PTL) 1).
According to the echo sounding apparatus so far, a transmitting interval is controlled so as not to transmit the next pulse before a reception echo is received in consideration of an underwater sound velocity of the ultrasonic wave, and the sounding is performed. As shown in FIG. 2, a sounding apparatus equipped with only one beam is called a single beam sounding apparatus and a sounding apparatus which has been proposed in recent years and in which a plurality of beams are spread in a fan shape as called a multibeam sounding apparatus (for example, refer to PTL 2). The multibeam sounding apparatus can measure depths in a wide range in a lump.
It is now assumed that a depth is equal to D and a transmitting interval of the transmission pulse is equal to T. When (2D/1500)<T, as shown in FIG. 3A, a time difference between the transmission pulse and the reception echo corresponds to (2D/1500). The depth can be measured from the time difference. However, when (2D/1500)≥T, as shown in FIG. 3B, the reception echo arrives after the next transmission pulse was transmitted. Therefore, to which one of the transmission pulses the reception echo corresponds cannot be known. A wrong depth is measured on the basis of such a time difference FD. It is, therefore, a condition of (2D/1500)<T is necessary hitherto.
Such a point that the transmitting period cannot be shortened results in that a resolution in the horizontal direction of the sounding cannot be decreased. A resolution of a measurement in the progressing direction (horizontal direction) of a ship will be described with reference to FIG. 4. A resolution ΔH (m) in the horizontal direction in the case of performing the sounding of a depth D (m) at a ship velocity V (m/sec) is obtained by the following equation.ΔH=VT>2DV/1500
For example, if the ship sails at 10 kt (speed per hour: 10×1.852 km) and the transmitting period is equal to 1 second, sounding data can be obtained only every about 5 m. To measure a sea bottom of a depth of 1000 m, it can be measured only when the transmitting period T is set to ((1000×2)/1500=1.33 seconds) or more. However, if the ship sails at 10 kt, the ship progresses by 6.7 m after 1.33 seconds. Therefore, the resolution ΔH of the measurement is equal to 6.67 m. Although the multibeam sounding apparatus can measure depths in a wide range in a lump, the resolution of the measurement in the progressing direction of the ship is similar to that in the case of the single beam.
In the conventional echo sounding apparatus, in order to raise the resolution of the measurement, there is only a method of reducing the velocity of the ship. Therefore, the conventional echo sounding apparatus has such a problem that in the case of raising the resolution in the horizontal direction of the sounding, a time required for the sounding becomes long.
Further, as shown in FIG. 5, in the case of measuring a sea bottom by a sound wave, a depth which is measured becomes deeper or shallower than the true sea bottom because a reference sea level is subjected to a ship motion of waves. The bottom depth which is obtained by the measurement is as shown in FIG. 6 and a distance to the true bottom cannot be measured. In order to solve such a problem, it is necessary to detect a component of the ship motion and correct the ship motion.
As mentioned above, in the conventional echo sounder, the transmitting period cannot be shortened and since the transmitting period is longer or almost equal to a period of the ship motion due to the waves, it is difficult that the ship motion due to an influence by the waves is detected and corrected. From a sampling theory, unless the ship motion is sampled at a frequency which is two or more times as high as a maximum value of a frequency component of the ship motion, it is impossible to detect the ship motion component. Therefore, in the case of correcting the ship motion, hitherto, as disclosed in PTL 3, generally, a displacement amount is detected from triaxial accelerations and the ship motion is corrected based on a detection result.